Fuel cells provide an environmentally friendly source of electrical current. One form of fuel cell used for generating electrical power, particularly for vehicle propulsion and for smaller scale stationary power generation, includes an anode channel for receiving a flow of hydrogen gas, a cathode channel for receiving a flow of oxygen gas, and a polymer electrolyte membrane (PEM) which separates the anode channel from the cathode channel. Oxygen gas which enters the cathode, reacts with hydrogen ions, which cross the electrolyte to generate a flow of electrons. Environmentally safe water vapor is produced as a byproduct.
External production, purification, dispensing and storage of hydrogen (either as compressed gas or cryogenic liquid) requires costly infrastructure, while storing of hydrogen fuel on vehicles presents considerable technical and economic barriers. Accordingly, for stationary power generation, it is preferred to generate hydrogen from natural gas by steam reforming or partial oxidation followed by water gas shift reaction. For fuel cell vehicles using a liquid fuel, it is preferred to generate hydrogen from methanol by steam reforming or from gasoline by partial oxidation or autothermal reforming, again followed by water gas shift reaction. However, the resulting hydrogen contains contaminants, such as carbon monoxide and carbon dioxide impurities, that cannot be tolerated respectively by the PEM fuel cell catalytic electrodes in more than trace levels.
The conventional method of removing residual carbon monoxide from the hydrogen feed to PEM fuel cells has been catalytic selective oxidation, which compromises efficiency as both the carbon monoxide and a fraction of the hydrogen are consumed by low temperature oxidation, without any recovery of the heat of combustion. Palladium diffusion membranes can be used for hydrogen purification, but have the disadvantages of delivering purified hydrogen at low pressure, and also the use of rare and costly materials.
Pressure swing adsorption systems (PSA) have the attractive features of being able to provide continuous sources of oxygen and hydrogen gas, without significant contaminant levels. PSA systems and vacuum pressure swing adsorption systems (VPSA) separate gas fractions from a gas mixture by coordinating pressure cycling and flow reversals over an adsorber or adsorbent bed, which preferentially adsorbs a more readily adsorbed gas component relative to a less readily adsorbed gas component of the mixture. The total pressure of the gas mixture in the adsorber is elevated while the gas mixture is flowing through the adsorber from a first end to a second end thereof, and is reduced while the gas mixture is flowing through the adsorbent from the second end back to the first end. As the PSA cycle is repeated, the less readily adsorbed component is concentrated adjacent the second end of the adsorber, while the more readily adsorbed component is concentrated adjacent the first end of the adsorber. As a result, a “light” product (a gas fraction depleted in the more readily adsorbed component and enriched in the less readily adsorbed component) is delivered from the second end of the adsorber, and a “heavy” product (a gas fraction enriched in the more strongly adsorbed component) is exhausted from the first end of the adsorber.
Numerous copper-based, CO-selective adsorbents have been disclosed by Rabo et al (U.S. Pat. No. 4,019,879), Hirai (U.S. Pat. No. 4,587,114), Nishida et al. (U.S. Pat. No. 4,743,276), Tajima et al. (U.S. Pat. No. 4,783,433), Tsuji et al. (U.S. Pat. No. 4,914,076), Xie et al. (U.S. Pat. No. 4,917,711), Golden et al. (U.S. Pat. Nos. 5,126,310; 5,258,571; and 5,531,809), and Hable et al. (U.S. Pat. No. 6,060,032). Use of some such CO-selective adsorbents in pressure swing adsorption processes for removal or concentration of CO has been commercially established at industrial scale.
Using certain adsorbents for removing CO from reformate for PEM fuel cells has been investigated by researchers at the Argonne National Laboratory, as reported in the 1998 annual report of the Fuel Cells for Transportation Program of the U.S. Department of Energy, Office of Advanced Transportation Technologies. Bellows (U.S. Pat. No. 5,604,047) discloses using selected noble metals, and the carbides and nitrides of certain metals, as carbon monoxide adsorbents in a steam displacement purge cycle for removing CO from reformate feed to fuel cells.
However, the conventional system for implementing pressure swing adsorption or vacuum pressure swing adsorption uses two or more stationary adsorbers in parallel, with directional valving at each end of each adsorber to connect the adsorbers in alternating sequence to pressure sources and sinks. This system is cumbersome and expensive to implement due to the large size of the adsorbers and the complexity of the valving required. Further, the conventional PSA system use of applied energy inefficiently because of irreversible gas expansion steps as adsorbers are cyclically pressurized and depressurized within the PSA process. Conventional PSA systems could not be applied to fuel cell power plants for vehicles, as such PSA systems are far too bulky and heavy because of their low cycle frequency and consequently large adsorbent inventory.
Another problem is the need for air compression with a substantial mechanical parasitic load to achieve high power density and high voltage efficiency with PEM fuel cells, either in the absence of PSA in prior art fuel cell systems, or to a lesser extent with the use of PSA to increase oxygen concentration. If, as usual by the case, mechanical power is provided by an electric motor powered by the fuel cell, significant efficiency losses occur in electrical power conversion and conditioning for variable speed compressor drive, and the fuel cell stack must be substantially larger to support this parasitic load as well as the application load to which useful power is delivered. In prior art PEM fuel cell power plants for automotive and other transportation applications, approximately 20% of the gross power output of the fuel cell is diverted to the parasitic load of air compression.
Yet another problem arises in the need to provide heat for endothermic fuel processing reactions to generate low purity reformate hydrogen from hydrocarbon fuels (e.g. natural gas, gasoline or diesel fuel) or oxygenate fuels (e.g. methanol, ethanol or dimethyl ether). In the prior art, the necessary heat for steam reforming of natural gas or methanol is provided least in part by burning hydrogen provided as anode tail gas from the fuel cell. Especially in the case of methanol reforming, which can be performed at relatively low temperature, combustion of valuable hydrogen to generate such low grade heat is extremely detrimental to overall energetic efficiency.
Likewise, the necessary heat for processing heavier fuels, such as gasoline, is achieved by combusting a portion of the fuel in a partial oxidation or autothermal reforming process. Again, a portion of the high-grade fuel is consumed to upgrade the remainder of that fuel to low purity hydrogen than can be purified for use in the fuel cell. With a low temperature fuel cell, thermal efficiency of prior art fuel processing systems has been extremely low, as high grade fuel is consumed. No opportunity has been found for efficient thermal integration between a high temperature fuel processor and a low temperature fuel cell in transport applications.
Combined cycle power plants with a gas turbine cycle integrated with a fuel cell system have been disclosed. Fuel cell auxiliary power units have been proposed for automobiles and passenger railcars with internal combustion engines as primary power plants. PCT Patent Application Publication No. WO 00/16425 provides examples of how PSA units may be integrated with gas turbine power plants, or with fuel cell power plants having a gas turbine auxiliary engine.